Growing energy demands and dwindling resources have increased demand for both alternative forms of energy and energy savings. For example, hydrogen and electric automobiles are now commercially available and increasing in popularity. Alternative fuel vehicles, such as these, typically require large energy stores (e.g., for acceleration), resulting in the ongoing development of capacitor and “super”-capacitor technologies. Of course these technologies have a broad range of applications including portable electronic devices, e.g., cell phones, lap top computers and personal digital assistants and are not limited to use in automobiles.
In general, an electrochemical capacitor with a large specific capacitance is produced by combining an electrode material having a large specific surface area with a material that can be reversibly oxidized or reduced over a wide potential range. Carbon powders and conducting polymers have a large double layer capacitance and multivalent metal oxides (e.g., ruthenium and iridium oxides) exhibit large Faradic pseudo capacitance. Accordingly, these materials have often been studied for application as “super”-capacitors. An amorphous phase of RuO2:XH2O formed by the sol-gel method at low temperatures shows a specific capacitance as high as 720 F/g in an acidic electrolyte, but the high cost of these materials detracts from their commercialization.
Electrochromic coatings are also being developed in an effort to promote energy savings. Applications include energy-efficient, dynamically-controlled (so-called “smart”) windows, sunroofs, as well as opto-electronic “shutters.” A variable transmittance window coating operated as part of an intelligent building energy management system can provide substantial energy savings. In particular, electrochromic windows may be used to control solar heat gain through windows and thereby reduce energy requirements, e.g., for air-conditioning in automobiles and buildings. Of course these technologies also have a broad range of other applications, e.g., anti-glare automobile rearview mirrors and other surfaces.
Inorganic electrochromic coatings operate by insertion of ionic species into a host lattice to effect changes in the optical properties. For example, amorphous tungsten oxide films appear transparent. Injecting lithium (or hydrogen) ions and electrons causes the film to absorb light and the color of the film to take on a dark blue appearance. This color change in the films is directly related to the double injection/extraction of electrons and ions in the films, which can be written in simplified form as:xM++xe−+a−WO3=a−MxWO3 where M=H, Li, etc.
When lithium (or others, e.g., sodium, potassium, hydrogen, etc.) ions and electrons are injected into amorphous tungsten oxide films, the electrons reduce W6+ ions to W5+. Lithium or hydrogen ions are necessary to satisfy the charge neutrality condition inside the film. However, ion insertion results in cyclic expansion and contraction of the bulk structure, and over time, deteriorates the electrochromic material and thus limits the operational life of an electrochromic device.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.